The present invention relates to a semiconductor device, and more particularly to a resin sealed semiconductor device.
A resin sealed semiconductor device is ordinarily assembled by mounting on a lead frame a semiconductor element which has circuit elements formed thereon, connecting the electrodes of the semiconductor element and the leads of the lead frame with bonding wires and sealing the semiconductor element and wires with a resin member. Accordingly, a resin sealed semiconductor device is constructed having components that may be roughly divided into an epoxy resin member which is a sealing material, a silicon substrate with circuit elements formed thereon and a metallic lead frame on which is mounted a silicon substrate. Since the coefficients of thremal expansion of these materials are mutually different, thermal stresses are generated at the respective boundaries of the epoxy resin member, the silicon substrate and the lead frame due to expansion or contraction that occurs due to changes in the temperature. In particular components of the circuit elements that are constructed on the silicon substrate surface are sometimes broken by stresses that act on the surface of the silicon substrate, resulting in non-use of the semiconductor device. For this reason, a temperature cycling test is usually undertaken in order to confirm whether the semiconductor device can withstand changes in the temperature.
The above-mentioned breaking phenomenon of components of the circuit elements will now be described in more detail.
During wire bonding, a bonding ball is formed at a part of the wire that is connected to a bonding pad on the silicon substrate. The bonding ball is given a shape that protrudes from the upper surface of the silicon substrate so that it is subjected most severely to the influence of the stresses in the epoxy resin when a temperature change takes place. As a result the bonding pad which is connected to the bonding ball suffers from large stresses. Of particular concern is the component of the stresses that acts in the direction parallel to the substrate surface, which generates a shear in the bonding pad and creates a change in its shape. The deformation thus created is spread to the circuit element of the output buffer circuit via a leader line which is connected to the bonding pad. Since the tip of the leader line is connected via contacts to a diffused layer on the silicon substrate, the spreading deformation in the leader line acts on the contacts as a concentration of stresses When the contacts can no longer withstand the stress concentration, the lead line is broken. Further, the stress concentration cause pressure on the wirings which are on a level lower than that of the lead line, thus causing disconnections of the lower level wirings
The conventional countermeasures to the above mentioned problems are as follows. Firstly, a resin with a coefficient of expansion made to match that of silicon is employed. This is obtained by blending various kinds of fillers with a resin, such as epoxy resin, which has a coefficient of thermal expansion close to that of silicon. Secondly, the width of the wirings in the portion receiving large stresses is increased, or the spacing between the wirings is increased
However, with regard to the first countermeasure, the sealing resin having a coefficient of expansion made to match that of silicon has an inferior moisture resistance creating a separate problem that the moisture resistance of the semiconductor device is deteriorated In the second countermeasure, the effort to improve the integration density of the semiconductor device is obstructed, its electrical capacitance increased, and the operating speed is reduced.